CI: Drop stale test-artifacts/alpine/3.18.dockerfile

This should have been dropped too, as the authoritative copy lives in another
repo now.

Fixes: babe11b46c ("CI: Drop alpine-3.18-rootfs-export and use test-artefacts")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
(cherry picked from commit 424d20afae)

automation/tests-artifacts/alpine/3.18.dockerfile

@@ -1,67 +0,0 @@
-# syntax=docker/dockerfile:1
-FROM --platform=linux/amd64 alpine:3.18
-LABEL maintainer.name="The Xen Project" \
-      maintainer.email="xen-devel@lists.xenproject.org"
-
-ENV USER root
-
-RUN mkdir /build
-WORKDIR /build
-
-RUN \
-  # apk
-  apk update && \
-  \
-  # xen runtime deps
-  apk add musl && \
-  apk add libgcc && \
-  apk add openrc && \
-  apk add busybox && \
-  apk add sudo && \
-  apk add dbus && \
-  apk add bash && \
-  apk add python3 && \
-  apk add zlib && \
-  apk add ncurses && \
-  apk add yajl && \
-  apk add libaio && \
-  apk add xz && \
-  apk add util-linux && \
-  apk add argp-standalone && \
-  apk add libfdt && \
-  apk add glib && \
-  apk add pixman && \
-  apk add curl && \
-  apk add udev && \
-  apk add pciutils && \
-  apk add libelf && \
-  \
-  # Xen
-  cd / && \
-  # Minimal ramdisk environment in case of cpio output
-  rc-update add udev && \
-  rc-update add udev-trigger && \
-  rc-update add udev-settle && \
-  rc-update add loopback sysinit && \
-  rc-update add bootmisc boot && \
-  rc-update add devfs sysinit && \
-  rc-update add dmesg sysinit && \
-  rc-update add hostname boot && \
-  rc-update add hwclock boot && \
-  rc-update add hwdrivers sysinit && \
-  rc-update add modules boot && \
-  rc-update add killprocs shutdown && \
-  rc-update add mount-ro shutdown && \
-  rc-update add savecache shutdown && \
-  rc-update add local default && \
-  cp -a /sbin/init /init && \
-  echo "ttyS0" >> /etc/securetty && \
-  echo "hvc0" >> /etc/securetty && \
-  echo "ttyS0::respawn:/sbin/getty -L ttyS0 115200 vt100" >> /etc/inittab && \
-  echo "hvc0::respawn:/sbin/getty -L hvc0 115200 vt100" >> /etc/inittab && \
-  echo > /etc/modules && \
-  passwd -d "root" root && \
-  \
-  # Create rootfs
-  cd / && \
-  tar cvzf /initrd.tar.gz bin dev etc home init lib mnt opt root sbin usr var